BACKGROUND: The dietary approaches to stop hypertension (DASH) diet have several components like high amounts of fruits, vegetables, low-fat dairy products, and vitamin C and low amounts of simple sugars that might improve attention-deficit hyperactivity disorder (ADHD) symptoms. We aimed to investigate the effect of a DASH diet on children (aged 6-12 years) with ADHD, for the first time. METHODS: Participants were randomized to receive a DASH or a control diet for 12 weeks. The severity of ADHD symptoms [determined by abbreviated 10-item Conner's scale (ACS), 18-item Swanson, Nolan and Pelham (SNAP-IV) scale and strengths and difficulties questionnaire (SDQ)] were assessed every four weeks. RESULTS: Eighty children completed the study. After adjustment for confounders, parent (- 4.71 for the DASH group vs. - 3 for the control group) and teacher-reported (- 5.35 vs. - 1.87) ACS scores, parent-, teacher-, child-reported hyperactivity (- 1.40 vs. - 0.66, - 1.95 vs. -0.63, - 1.60 vs. - 0.43, respectively), emotional symptoms (- 1.50 vs. - 0.45, - 1.42 vs. - 0.63, and - 1.09 vs. - 0.61, respectively), and total SDQ scores (- 3.81 vs. - 1.65, - 4.11 vs. - 1.23, - 4.44 vs. - 1.26, respectively), teacher-reported of conduct problems (- 1.42 vs. - 0.63), peer relationship problems (- 0.87 vs. - 0.07), and prosocial behaviors (1.36 vs. 0.08) assessed by the SDQ were significantly improved in the DASH group compared with the control group (P < 0.05). CONCLUSION: Adherence to a DASH-style diet might improve ADHD symptoms. Further RCTs which include participants from both sexes and with longer follow-up periods are needed to warrant current findings (The trial registration code: IRCT20130223012571N6; http://irct.ir/trial/12623 ). Trial registration Trial registration number: The trial was registered in the Iranian registry of clinical trials (registration code: IRCT20130223012571N6), URL: http://irct.ir/trial/12623 .